1. Field of the Invention
The present invention relates to a time acquisition apparatus to acquire current time using standard radio wave and a radio wave clock on which the time acquisition apparatus is mounted.
2. Description of Related Art
Currently, a long-wave standard time radio wave is transmitted from transmitting stations in various countries such as Japan, Germany, England and Switzerland For example, in Japan, the standard time radio waves of 40 kHz and 60 kHz that have been subjected to amplitude modulation are respectively transmitted from transmitting stations in Hukushima prefecture and Saga prefecture.
The standard time radio wave includes a code sequence which constructs a time code indicating date and time, and is sent in 60 seconds per period. In other words, the period of the time code is 60 seconds.
A clock (radio wave clock) which receives such standard time radio wave including the time code to extract the time code from the received standard time radio wave so as to correct time has been put to practical use.
A receiving circuit of the radio wave clock includes: a band path filter (BPF) to receive the standard time radio wave received by an antenna to extract only a standard time radio wave signal; a demodulating circuit to demodulate the standard time radio wave signal that has been subjected to amplitude modulation by envelope detection and the like; and a processing circuit to read out the time code included in the signal demodulated by the demodulating circuit.
A conventional processing circuit synchronizes a starting point of a timekeeping period for data discrimination with a rising edge of the demodulated signal, and then binarizes the demodulated signal with a predetermined sampling period to acquire TCO data which is a binary bit sequence. Moreover, the processing circuit measures a pulse width (namely, a time of bit “1”, or a time of bit “0”) of the TCO data to determine any one of code “P”, “0” and “1” according to the width size so as to acquire time information based on determined code sequence.
The conventional processing circuit passes through processes including a second bit synchronization processing, a minute bit synchronization processing, code loading, and consistency judging, from starting reception of the standard time radio wave to acquiring the time information When processing is not properly completed in each of the processes, the processing circuit needs to start the processing again from the beginning.
Thus, the processing sometimes needs to be started again many times due to noise included in the signal, and time to acquisition of the time information sometimes becomes seriously long.
The second bit synchronization is to detect a rising edge of the code which comes per one (1) second among the code indicated by the TCO data. By repeating the second bit synchronization, a portion where a position marker “P0” provided at ending of a frame and a marker “M” provided at beginning of the frame are located consecutively can be detected. This consecutive portion comes every one (1) minute (60 seconds). A position of the marker “M” locates in data of the beginning frame among the TCO data. Detecting the marker “M” is hereinafter called the minute bit synchronization.
Since the beginning of the frame is recognized by the above-described minute bit synchronization, then the code loading is started, and after one (1) frame of data is obtained, a parity bit is examined to judge whether or not the data has impossible value (value which can not be real data and time) (the consistency judging). For example, the minute bit synchronization sometimes requires 60 seconds for finding the beginning of the frame. Of course it requires several fold longer time than above time in order to detect the beginning of the frame across several frames.
In US2005/0195690A1, the TCO data is obtained by binarizing the demodulated signal at predetermined sampling intervals (50 ms), and data constellation composed of binary bit sequences is listed, each of the binary bit sequences corresponding to one (1) second (20 samples).
An apparatus disclosed in US2005/0195690A1 compares above bit sequence with a template of the binary bit sequence indicating code “P: position marker”, a template of the binary bit sequence indicating code “1”, and a template of the binary bit sequence indicating code “0” respectively, to obtain their correlation, and judges which of codes “P”, “1” and “0” the bit sequence corresponds to, based on the correlation.
A technique disclosed in US2005/0195690A1 acquires the TOC data which is the binary bit sequence to perform matching with the template. Under a condition that electric field intensity is weak or that much noise is mixed into the demodulated signal, the acquired TCO data would include many errors. Therefore, it was necessary to fine-adjust a threshold of a filter for removing noise from the demodulated signal or an AD converter so as to improve quality of the TCO data.